I once read/watch that if you get some happiness, someone out there is less happy because of it. Imagine happiness in this world has fixed amount and is shared between people. Therefore, if some people get too much of it, the others will get less as the result. 
Here we see that in order to make sensei happy again, they need to took away a bit of happiness from someone, which accidentally was Suzu. Can someone pat her head now? 
Moral of the story: Don't be greedy and be grateful to God for whatever you have.